1. Field of the Invention
This invention relates to the manufacture of electronic circuits and more particularly to the manufacture of integrated circuits and printed circuit boards by patterning layers in succession to form features which comprise elements of an integrated circuit or a printed circuit board.
2. Description of the Relevant Art
In the manufacture of electronic circuits, one or more features are often formed by removing unwanted portions of a layer of a desired material. For example, printed circuit boards, also called printed wiring boards, are typically manufactured by first depositing a layer of a metal (e.g., copper) upon an entire major surface of a flat sheet of an insulating material (e.g., epoxy-fiberglass composite). Unwanted portions of the metal layer are then removed in order to form the familiar conductive metal traces which provide point-to-point electrical connections. The process of removing unwanted portions of a layer of a desired material is called patterning. Integrated circuits are also manufactured by patterning layers of desired materials to form features upon substantially flat surfaces.
A layer of a desired material is typically patterned using a process called photolithography. Photolithography involves forming a protective layer of a light-sensitive material over the entire exposed surface of the layer to be patterned. The layer of the light-sensitive material is patterned by passing light through a mask in order to transfer a desired pattern to the light-sensitive material. Exposure to light either makes portions of the light-sensitive material soluble in a developing liquid (e.g., a positive photoresist material) or making the exposed portions insoluble in the developing liquid (e.g., a negative photoresist material). Following exposure, the layer of the light-sensitive material is developed in a manner analogous to the developing of exposed photographic film. The soluble portions of the layer of light-sensitive material directly overlying the unwanted portions of the underlying layer to be patterned are removed by immersion in the developing liquid, exposing the unwanted portions of the underlying layer. The exposed unwanted portions of the underlying layer are then removed during a subsequent etch step. Following the etch step, the remaining portions of the layer of light-sensitive material are removed, leaving one or more features formed from the layer of the desired material.
Due to the physical size of a typical printed circuit board (PCB), a process called proximity printing is commonly used to transfer a desired pattern to a layer of a light-sensitive material formed over a metal layer of the PCB. In proximity printing, the mask is the same size as the pattern being formed. The mask is placed between a light source and the PCB during exposure such that the mask is in close proximity to the light-sensitive layer.
The microscopic feature sizes of integrated circuits typically preclude proximity printing. Integrated circuit features are commonly formed using a process called projection printing. In projection printing, lens elements or mirrors are used to form an image of the mask such that the image is much smaller than the mask itself. The mask image is cast upon the surface of a layer of a lightsensitive material formed over a layer of a material to be patterned.
Semiconductor wafer fabrication involves forming multiple identical integrated circuits upon the surface of a single semiconductor wafer. The surface of the semiconductor wafer is divided into separate die areas, and an integrated circuit is formed within each die area. Common step-and-repeat projection exposure systems cast a mask image within each die area, one after another, until portions of a layer of a light-sensitive material covering all die areas have been exposed. A mask used in such a step-and-repeat projection exposure system is often called a "reticle". As used herein, the term "mask" will be used to describe a tool containing one or more patterns and used to selectively expose portions of a layer of a light-sensitive material to light. It is noted that this definition encompasses the term "reticle".
A mask used in photolithography is typically a glass plate having clear and opaque regions. The clear regions allow light incident upon one side of the mask to pass through the mask. The opaque regions block incident light, preventing light from passing through the mask. The opaque regions are typically made up of features formed from a layer of an opaque material such as emulsion, chrome, or iron oxide.
Masks are made by a detailed pattern transfer process. First, a layer of a desired opaque material is formed over a major surface of a glass plate. A layer of a light-sensitive or charge-sensitive material is then formed over the entire exposed surface of the opaque layer. In the case of a layer of a light-sensitive material, the layer is exposed by repeatedly projecting relatively small polygons (e.g., rectangles) of light upon the surface of the light-sensitive layer, overlapping the projections in order to transfer a desired pattern to the light-sensitive layer. A layer of a charge-sensitive material is exposed by scanning the entire surface of the charge-sensitive layer with an electron gun capable of producing a relatively narrow beam of electrons. Selected regions of the charge-sensitive material are exposed to a beam of electrons during scanning in order to transfer a desired pattern to the charge-sensitive layer. Following exposure, the layer of the light-sensitive or charge-sensitive material is developed, exposing the unwanted portions of the underlying layer of the opaque material. The unwanted portions of the underlying layer of the opaque material are then removed by etching. Following the etch step, the remaining portions of the layer of light-sensitive or charge-sensitive material are removed, leaving opaque features formed from the layer of the opaque material.
The pattern transfer process used to define the clear and opaque regions of a mask requires accuracy, precision, and time. As a result, masks are expensive and require a certain amount of time to produce. In addition, masks reflect a selected circuit layout, and generally cannot be modified. Fabrication of a modern integrated circuit typically requires 10 or more combined layering and patterning (i.e., masking) steps, each requiring a unique mask. A change to a design of an integrated circuit may require the fabrication of several new masks in order to implement the design change. In addition, masks do not promote integrated circuit layout experimentation in order to evaluate alternate circuit layouts in terms of circuit performance.
Liquid crystal displays (LCDs) are commonly used in calculators, watches, hand-held games, portable television sets, portable computer monitors, and projection display systems. A typical LCD includes a liquid crystal material sandwiched between two glass plates. Multiple light control elements of an LCD selectively pass or block light in response to an electrical signal (i.e., an applied electric field). Calculators and watches include a linear array of numerical LCDs. Each numerical LCD is capable of displaying a decimal character between `0` and `9`. Each numerical LCD includes seven light control elements called "segments". A decimal character is displayed when two or more segments of a numerical LCD are active; only two active segments are required to display the number `1`, while all seven segments must be active to display the number `8`.
LCDs used in portable televisions, portable computer monitors, hand-held games, and projection display systems are graphical LCDs. A graphical LCD includes a large number of identical light control elements called "pixels" arranged in a two-dimensional array. A graphical LCD displays characters (e.g., numbers and letters) and graphical representations by activating several pixels simultaneously.
LCD projection display systems include LCD panels having pixels which selectively block light or allow light to pass through the panel. In a black-and-white LCD panel, a pixel receiving an electrical signal (i.e., an active pixel) typically blocks light. Adjacent inactive pixels form "clear" (i.e., transparent) regions which allow light to pass through the LCD panel. Substantially opaque regions are formed where several adjacent pixels of the LCD panel are active. It would be advantageous that a pattern be displayed upon an LCD panel which would desirably allow the LCD panel to function as a mask. Unlike a mask, however, the pattern formed on an LCD panel may be changed in a fraction of a second simply by altering the electrical display signals. Currently, 10 in..times.10 in. LCD panels are available which have over 1,000,000 pixels arranged in two-dimensional arrays of 1,000+ pixels by 1,000+ pixels.
It would thus be desirable to use an LCD panel as a configurable mask for photolithography. Such a mask may be quickly and easily modified, allowing design changes to be implemented in a fraction of the time currently required to fabricate one or more conventional masks. In addition, the expense involved in effecting such a design change is minimal. Furthermore, such an easily modifiable mask would promote circuit layout experimentation in search of improved circuit performance.